1. Genes That Affect the Assembly of the Nervous System in Drosophila Embryos. RNA interference (RNAi) was used to screen 3,314 Drosophila double-stranded RNAs, corresponding to approximately 25 percent of Drosophila genes, for genes that affect the development of the embryonic nervous system. RNAi mediated gene silencing in Drosophila embryos resulted in loss of function mutant phenotypes for 43 genes, which is 1.3 percent of the genes that were screened. Eighteen genes were found that were not known previously to affect the development of the nervous system. Some of the most interesting genes found have unknown functions; whereas, other genes encode protein kinases, transcription factors, signaling proteins, as well as proteins with other functions. 2. vnd/NK-2 Homeobox Gene. A transient transfection assay using Drosophila S2 tissue culture cells and wild-type and mutant Drosophila vnd/NK-2 homeobox cDNA?s was used to localize repression and activation domains of vnd/NK-2 homeodomain protein. A repression domain was identified near the N-terminus of vnd/NK-2 homeodomain protein (amino acid residues 154-193), which contains many hydrophobic amino acid residues. The major determinants of the repression domain were shown to be amino acid residues F155, W158, I161, L162, L163, and W166. Truncated protein consisting of the N-terminal repression domain and the DNA-binding homeodomain repressed transcription as efficiently as wild-type vnd/NK-2 protein. An activation domain was identified between the tinman domain and the homeodomain (amino acid residues 277-543), which consists of a glutamine-rich subdomain and two acidic subdomains. No effect was detected of the tinman domain or the NK-2 specific domain on either activation or repression of a b-galactosidase reporter gene.